In vivo mapping of local cerebral blood flow by xenon-enhanced computed tomography

Sinking Skin Flap Syndrome

Sinking skin flap syndrome is defined as a series of neurologic symptoms with the skin depression at the site of cranial defect, which develop several weeks to months after large external cerebral decompression.The case of a 28-year-old female with the sinking skin flap syndrome is reported together with the evaluation of cerebral blood flow using xenon computed tomography (CT). Although her general condition stabilized within 7 months after the injury, the skin of the bilateral temporal regions was markedly depressed due to large bone defects. She was confined to bed and showed reduced levels of consciousness. We decided to treat this case by performing cranioplasty with a hydroxyapatite ceramic implant. Not only were good cranial contour reconstructed after cranioplasty, but neurologic conditions were also improved after cranioplasty. Regarding the change in cerebral blood flow in the present case, as measured with xenon CT, the cerebral blood flow 3 days after the injury was 18.7 +/- 12.3 mL/100 mL/min and 26.5 +/- 11.6 mL/100 mL/min in the left and right hemispheres, respectively. After the bilateral cranioplasty, it had increased by approximately 2-fold to 36.4 +/- 23.2 mL/100 mL/min in the left hemisphere and approximately 1.5-fold to 43.8 +/- 23.3 mL/100 mL/min in the right hemisphere as compared with the levels obtained 3 days after the injury.Therefore, xenon CT appears to be useful in the monitoring of regional cerebral blood flow in patients with cranial bone defects that are directly affected by atmospheric pressure and in predicting functional prognosis. For the sinking skin flap syndrome cases, cranioplasty is not only useful for cerebral protection and improvement of appearance, but cranioplasty is also useful for improving neurologic symptoms.

Xenon-Inhalation Computed Tomography for Noninvasive Quantitative Measurement of Tissue Blood Flow in Hepatocellular Carcinoma

OBJECTIVE

The purpose of this study was to separately measure the arterial and portal venous tissue blood flow (TBF) of hepatocellular carcinoma (HCC) with a noninvasive method using xenon inhalation CT (xenon-CT) and to differentiate between well-differentiated HCCs and moderately and poorly differentiated HCCs.

MATERIALS AND METHODS

Total, arterial and portal venous TBFs of 38 surgically proven HCC nodules from 38 patients were measured by means of xenon-CT. Serial abdominal CT scans were obtained before and after inhalation of nonradioactive xenon gas. TBF was computed using the Fick principle, after which the correlation between TBF and pathologic features of the tumors was determined.

RESULTS

Total, arterial, and portal venous TBFs of HCC were 125.7 +/- 59.9 mL/min/100g, 102.5 +/- 37.3, and 22.2 +/- 11.4, respectively, and the corresponding findings for hepatic parenchyma were 67.3 +/- 13.1, 25.2 +/-9.6, and 42.4 +/- 11.0. Total and arterial TBFs of HCC were significantly higher than those of the hepatic parenchyma (P < 0.01), whereas portal venous TBF of HCC was significantly lower than that of hepatic parenchyma (P < 0.01). Arterial TBF of moderately or poorly differentiated HCC (120.4 +/- 38.2) was significantly higher than that of well-differentiated HCC (60.4 +/- 43.5) (P < 0.01).

CONCLUSIONS

Arterial and portal venous TBFs of HCC could be measured separately, noninvasively, and safely with xenon-CT. Correlation between TBF and pathologic features of tumors indicate that xenon-CT can be used to differentiate between well-differentiated HCCs and moderately and poorly differentiated HCCs.

Evidence-based neuroimaging in acute ischemic stroke

The imaging work-up of patients with acute neurologic deficits should begin with noncontrast CT to exclude intracerebral hemorrhage. Based on positive results from the NINDS t-PA trial, the overriding objectives of imaging in the selection of patients for t-PA treatment are the detection of hemorrhage and rapid evaluation (speed of imaging). Despite its limited sensitivity for the identification of an ischemic stroke lesion, CT has multiple advantages over MR imaging in the initial diagnostic work-up. Advanced MR techniques promise to provide anatomic, physiologic, and vascular information in a single examination, and the ability to increase treatment specificity and improve outcome. Clinical outcome data are lacking; therefore, the routine use of screening MR imaging before t-PA therapy is not supported. Rigorous validation and correlation to clinical outcomes will be required.

The Cortical Ischemic Core and Not the Consistently Present Penumbra Is a Determinant of Clinical Outcome in Acute Middle Cerebral Artery Occlusion

BACKGROUND AND PURPOSE

Patient selection for acute stroke therapy based on physiology rather than on time may lead to expansion of the therapeutic window, improved outcomes, and fewer side effects than currently achieved. This approach requires early determination of both irreversible (core) and reversible (penumbra) ischemia in acute stroke.

METHODS

Using established perfusion thresholds, we characterized the relationship among core, penumbra, and brain tissue perfused above penumbral thresholds (non-core/non-penumbra [NC/NP]) in 36 patients with middle cerebral artery (MCA) stem occlusion who underwent quantitative cerebral blood flow (CBF) assessment with xenon-enhanced CT within 6 hours of symptom onset.

RESULTS

While great variability in the mean+/-SD percentage of core (37.6+/-18.7) and NC/NP (30.3+/-16.6) was observed, the percentage of penumbra was relatively constant from individual to individual, constituting approximately one third of the cortical MCA territory (32.1+/-7). In univariable and multivariable analyses, percent core and not percent penumbra was significantly associated with outcome.

CONCLUSIONS

In acute MCA occlusion, penumbra is consistently present within a relatively narrow range, despite great variability in the size of core. This may explain why the core and not the penumbra is the main determinant of outcome in our group of patients. Recanalization therapy in acute MCA occlusion should ideally be guided by diagnostic methods capable of rapidly and reliably identifying irreversible ischemia.

Quantitative multilevel mapping of hepatic blood flow by xenon computed tomography using aorta

A noninvasive and quantitative technique has been developed to measure human hepatic blood flow by xenon computed tomography (Xe-CT). Accurate data on time-dependent xenon concentrations in the arterial blood are indispensable for Xe-CT to ensure quantitativeness of measured blood flow. A method has been established by our group to use both aorta and end-tidal data to obtain arterial xenon information. Multilevel (3 levels) maps of arterial blood flow (Fa), portal blood flow (Fp), and partition coefficient (lambda) were created for patients with chronic hepatitis. A method to objectively evaluate Fa, Fp, and lambda values for the whole liver has also been developed by our group.

Temporal resolution and the evaluation of candidate algorithms for four-dimensional CT

The four-dimensional computed tomography ("4D-CT") with area detector has been developed for dynamic volumetric imaging with large longitudinal coverage. In this paper one of the key technologies for 4D-CT development is discussed: Image reconstruction algorithm with high temporal resolution. All of the cone-beam algorithms investigated previously assume that the object is stationary. In this paper a new class of cone-beam problem is addressed: a dynamic volumetric (4-D) imaging. A continuously rotating circular (stationary couch) scanning is employed, and then, a generalized version of the well-known Feldkamp algorithm with the following three steps is performed: (1) applying a weighting function (along the time axis) to projection data, (2) filtering the weighted data along the detector row direction, (3) cone-beam backprojecting of the filtered data along the corresponding x-ray path. The weighting function controls the time center, the temporal resolution, and the image quality. Four weighting functions developed for fan-beam reconstruction were applied to the first step: (a) a constant weight fixed at 0.5 (FS-FDK), (b) feathering both edges of the (time) window (OS-FDK), (c) Parker's weight for a half-scan (HF-FDK), and (d) an extended Parker's weight, which allows us to use a larger range of projection data up to one rotation (NHS-FDK). We evaluated them in terms of temporal resolution, image noise, and image quality. Also, the cause of the artifact has been investigated. The temporal resolution of NHF-FDK equals that of HS-FDK, which is half of the one rotation period. For the moving object, NHS-FDK offers the best image quality. The images with FS-FDK are degraded by streak artifacts; HS-FDK provides poor image quality with good temporal resolution; and images by OS-FDK are blurred due to insufficient temporal resolution. The cause of the artifact was found as an inconsistency of projection data due to object motion (in FS-FDK) and lost 3-D-Radon data caused by applying Parker's weight (in HS-FDK). A hand toy was employed for the preliminary evaluation of dynamic volumetric imaging with the real 256-slice scanner. In an overall evaluation, NHS-FDK provides the stable and the sufficient image quality both with moving and stationary objects.

Steal affecting the central nervous system

Steal is a pathophysiological process in which increased blood flow through a low-resistance vascular bed is sufficient to divert flow away from a region of the central nervous system. Three disease states in which steal may cause neurological deficits due to central nervous system ischemia are reviewed. Subclavian steal occurs when stenosis of the subclavian artery proximal to the vertebral origin causes retrograde flow in the left vertebral artery. Patients with anatomic subclavian steal usually do not develop neurological symptoms but may rarely present with posterior circulation ischemia. Arteriovenous malformations alter cerebral blood flow patterns and regional perfusion pressure. It has been hypothesized that cerebral arteriovenous malformations may cause neurological deficits due to steal and that these deficits may be cured with arteriovenous malformation treatment. Intra-arterial pressure measurements and transcranial velocity studies show regional hemodynamic alterations. However, these changes have not been correlated with presenting symptoms. Evidence from single-photon emission computed tomography does suggest a relationship between regional hypoperfusion and neurological deficits. Coarctation of the aorta may divert flow from the spinal cord circulation through intercostal arteries distal to the stenosis. This is a possible but unproven mechanism of myelopathology. Steal syndromes may be amenable to treatment by open surgical or endovascular approaches. Experimental studies of the pathophysiology of steal are strengthened by precise definitions of the measured parameters and innovative applications of technology.

Focal cerebral hyperemia in postconcussive amnesia

Transient amnesia caused by minor head injury is commonly encountered in daily neurosurgical practice, but the mechanism of such amnesia has not been extensively studied. We measured the regional cerebral blood flow (rCBF) of patients with postconcussive amnesia with Xe/CT CBF to examine whether a focal disturbance of CBF exists. The Xe/CT CBF study was performed in eight patients with closed head injury without organic cerebral lesion while they were suffering from posttraumatic amnesia (concussion group). The time interval between accident and CBF measurement was less than 2 h in three patients, 5-6 h in two, 8-9 h in two, and 18 in one. The results were compared with those of nine normal volunteers and eight other age-matched patients who recovered without any neurological deficit despite the presence of hemorrhagic regions (mild hemorrhage group). The rCBF of the concussion group was significantly elevated in the bilateral mesial temporal cortex in comparison to the normal group. The rCBF in the mild hemorrhage group was lower than that of normal controls in all regions. The analysis of right-left difference in CBF indicated that there was significant asymmetry (right > left) in the frontal and temporal cortex in the concussion group, but not in the normal and mild hemorrhage group. This Xe/CT CBF study in acute stages of cerebral concussion, in which patients were amnestic, detected focal cerebral hyperemia. Such hyperemia in regions closely related to human memory function may be the result of vasoparalysis or the compensatory activation of memory circuits after denervation injury.

Ischemic penumbra: evidence from functional imaging in man

The ischemic penumbra is defined as tissue with flow within the thresholds for maintenance of function and of morphologic integrity. Penumbra tissue has the potential for recovery and therefore is the target for interventional therapy in acute ischemic stroke. The identification of the penumbra necessitates measuring flow reduced less than the functional threshold and differentiating between morphologic integrity and damage. This can be achieved by multitracer positron emission tomography (PET) and perfusion-weighted (PW) and diffusion-weighted magnetic resonance imaging (DW-MRI) in experimental models, in which the recovery of critically perfused tissue or its conversion to infarction was documented in repeat studies. Neuroimaging modalities applied in patients with acute ischemic stroke--multitracer PET, PW- and DW-MRI, single photon emission computed tomography (SPECT), perfusion, and Xe-enhanced computed tomography (CT)-- often cannot reliably identify penumbra tissue: multitracer studies for the assessment of flow and irreversible metabolic damage usually cannot be performed in the clinical setting; CT and MRI do not reliably detect irreversible damage in the first hours after stroke, and even DW-MRI may be misleading in some cases: determinations of perfusion alone yield a poor estimate of the state of the tissue as long as the time course of changes is not known in individual cases. Therefore, the range of flow values in ischemic tissue found later, either within or outside the infarct, was rather broad. New tracers--for example, receptor ligands or hypoxia markers--might improve the identification of penumbra tissue in the future. Despite these methodologic limitations, the validity of the concept of the penumbra was proven in several therapeutic studies in which thrombolytic treatment reversed critical ischemia and decreased the volume of final infarcts. Such neuroimaging findings might serve as surrogate targets in the selection of other therapeutic strategies for large clinical trials.

Correlation of regional cerebral blood flow measured by stable xenon CT and perfusion MRI

PURPOSE

The purpose of this work was to investigate the validity of perfusion MRI in comparison with stable xenon CT for evaluating regional cerebral blood flow (rCBF).

METHOD

The rCBF was measured by xenon CT and perfusion MRI within a 24 h interval in 10 patients (mean +/- SD age 63 +/- 10 years). For perfusion MRI, absolute values of rCBF were calculated based on the indicator dilution theory after injection of 0.1 mmol/kg of Gd-DTPA. Eight to 10 regions of interest (37 mm2) were located in the white and gray matter on the rCBF images for each of the 10 patients.

RESULTS

The mean +/- SD values of rCBF in gray matter were 48.5 +/- 14.1 ml/100 g/min measured by xenon CT and 52.2 +/- 16.4 ml/100 g/min measured by perfusion MRI. In the white matter, the rCBF was 22.6 +/- 9.1 ml/100 g/min by xenon CT and 27.4 +/- 6.8 ml/100 g/min by perfusion MRI. There was a good correlation of rCBF values between perfusion MRI and xenon CT (Pearson correlation coefficient 0.83; p < 0.0001).

CONCLUSION

Comparable to xenon CT, perfusion MRI provides relatively high resolution, quantitative local rCBF information coupled to MR anatomy.

The effect of reperfusion therapy on cerebral blood flow in acute stroke

The effect of reperfusion therapy on cerebral blood flow (CBF) in acute cerebral ischemia was studied using xenon-enhanced computed tomography (XeCT). The XeCT CBF studies of 10 patients were evaluated before and after thrombolytic therapy. CBF evidence of reperfusion was evaluated in relation to the angiographic results and the clinical outcomes. Six patients had occlusions of the middle cerebral artery and four of the internal carotid artery. The mean CBF of the ischemic areas before attempted reperfusion was 9 +/- 3 mL/100g/min compared with 34 +/- 9 mL/100g/min in the contralateral asymptomatic region (P<.001). Intra-arterial-thrombolysis was performed in nine patients, and in one patient the intravenous route was used. Reperfusion of the ischemic region was shown in 9 of 10 patients, both angiographically and with the XeCT CBF studies (the mean CBF increased from 9 +/- 3 mL/100g/min to 32 +/- 10 mL/100g/min, P<.001). Among the nine successfully reperfused patients, seven were neurologically improved, one was unchanged, and one died. The mean National Institutes of Health stroke scale in the eight reperfused survivors was 12 on admission and decreased to 6 on discharge. XeCT CBF measurements are correlated with the angiographic results and can assist in the understanding of the effects of thrombolytic therapy on CBF in acute stroke. Re-establishment of CBF is associated with an improved clinical outcome but exceptions can be found. Reperfusion can occur in ischemic brain regions even with very low CBF (approaching 0 mL/100g/min) although it is not associated with prevention of infarction.

Relationship between cerebral blood flow and the development of swelling and life-threatening herniation in acute ischemic stroke

OBJECT

The purpose of this study was to determine whether cerebral blood flow (CBF) measurements in acute stroke could be correlated with the subsequent development of cerebral edema and life-threatening brain herniation.

METHODS

Twenty patients with aggressively managed acute middle cerebral artery (MCA) territory strokes who underwent xenon-enhanced computerized tomography (Xe-CT) CBF scanning within 6 hours of onset of symptoms were retrospectively reviewed. The relationship among CBF and follow-up CT evidence of edema and clinical evidence of brain herniation during the 36 to 96 hours following stroke onset was analyzed. Initial CT scans displayed abnormal findings in 11 patients (55%), whereas the Xe-CT CBF scans showed abnormal findings in all patients (100%). The mean CBF in the symptomatic MCA territory was 10.4 ml/100 g/minute in patients who developed severe edema compared with 19 ml/100 g/minute in patients who developed mild edema (p < 0.05). The mean CBF in the symptomatic MCA territory was 8.6 ml/100 g/minute in patients who developed clinical brain herniation compared with 18 ml/100 g/minute in those who did not (p < 0.01). The mean CBF in the symptomatic MCA territory that was 15 ml/100 g/minute or lower was significantly associated with the development of severe edema and herniation (p < 0.05).

CONCLUSIONS

Within 6 hours of acute MCA territory stroke, Xe-CT CBF measurements can be used to predict the subsequent development of severe edema and progression to clinical life-threatening brain herniation. Early knowledge of the anatomical and clinical sequelae of stroke in the acute phase may aid in the triage of such patients and alert physicians to the potential need for more aggressive medical or neurosurgical intervention.

Quantitative cerebral blood flow determinations in acute ischemic stroke. Relationship to computed tomography and angiography

BACKGROUND AND PURPOSE

The advent of new modalities to treat acute ischemic stroke presents the need for accurate, early diagnosis. In acute ischemic stroke, CT scans are frequently normal or reveal only subtle hypodense changes. This study explored the utility and increased sensitivity of xenonenhanced CT (XeCT) in the diagnosis of acute cerebral ischemia and investigated the relationship between cerebral blood flow (CBF) measurements and early CT and angiographic findings in acute stroke.

METHODS

The CT scans, XeCT scans, and angiograms of 20 patients who presented within 6 hours of acute anterior circulation ischemic strokes were analyzed.

RESULTS

CT scans were abnormal in 11 (55%) of 20 patients. XeCT scans were abnormal in all 20 (100%) patients, showing regions of interest with CBF < 20 (mL/100 g per minute) in the symptomatic middle cerebral artery (MCA) territories. The mean CBF in the symptomatic MCA territories was significantly lower than than of the asymptomatic MCA territories (P < .0005). In patients with basal ganglia hypodensities, the mean symptomatic MCA territory CBF was significantly lower than that of patients who did not exhibit these early CT findings (P < .05). The mean symptomatic MCA territory CBF in patients with angiographic M1 occlusions was significantly lower than that of patients whose infarcts were caused by MCA branch occlusions (P < .01).

CONCLUSIONS

These results show that XeCT is more sensitive than CT in detecting acute strokes and that CBF measurements correlate with early CT and angiographic findings. XeCT may allow for the hyperacute identification of subsets of patients with acute ischemic events who are less likely to benefit and more likely to derive complications from aggressive stroke therapy.

Understanding functional neuroimaging methods based on neurovascular coupling

Functional neuroimaging techniques are usually grouped according to the employed apparatus into functional magnetic resonance imaging techniques (fMRI), nuclear medicine approaches such as single photon emission tomography (SPET) or positron emission tomography (PET), and optical approaches (measurement of intrinsic signals, near infrared spectroscopy (NIRS)). However, the physiological parameters that are measured with these methods do not necessarily follow this technical classification. On the one hand, using different imaging modalities the same physiological parameters are measured and on the other hand, using the same imaging devices completely different physiological parameters can be assessed. The present article covers those functional neuroimaging methods which measure the vascular response to functional brain activation (PET, SPET, fMRI and NIRS). First, starting with the traditional grouping of these methods, it is outlined how the specific methods assess vascular changes associated with brain activation in order to localize brain function. Based on the understanding of the underlying physiological events, subsequently, a new classification of functional neuroimaging methods is proposed.

Cerebral blood flow and vasoresponsivity within and around cerebral contusions

There is increasing evidence that regional ischemia plays a major role in secondary brain injury. Although the cortex underlying subdural hematomas seems particularly vulnerable to ischemia, little is known about the adequacy of cerebral blood flow (CBF) or the vasoresponsivity within the vascular bed of contusions. The authors used the xenon-enhanced computerized tomography (CT) CBF technique to define the CBF and vasoresponsivity of contusions, pericontusional parenchyma, and the remainder of the brain 24 to 48 hours after severe closed head injury in 10 patients: six patients with one contusion and four with two contusions, defined as mixed or high-density lesions on CT scanning. The CBF within the contusions (29.3 +/- 16.4 ml/100 g/minute, mean +/- standard deviation) was significantly lower than both that found in the adjacent 1-cm perimeter of normal-appearing tissue (42.5 +/- 15.8 ml/100 g/minute) and the mean global CBF (52.5 +/- 17.5 ml/100 g/minute) (p < 0.004, repeated-measures analysis of variance). A subset of seven patients (10 contusions) also underwent a second Xe-CT CBF study during mild hyperventilation (a PaCO2 of 24-32 mm Hg). In only two of these 10 contusions was vasoresponsivity less than 1% (range 0%-7.6%); in the rim of normal-appearing pericontusional tissue, it was 0.4% to 9.1%. The authors conclude that CBF within intracerebral contusions is highly variable and is often above 18 ml/100 g/minute, the reported threshold for irreversible ischemia. Intracontusional CBF is significantly reduced relative to surrounding brain parenchyma, and CO2 vasoresponsivity is usually present. In the contusion and the surrounding parenchyma, vasoresponsivity may be nearly three times normal, suggesting hypersensitivity to hyperventilation therapy. Given this possible hypersensitivity and relative hypoperfusion within and around cerebral contusions, these lesions are particularly vulnerable to secondary injury such as that which may be caused by hypotension or aggressive hyperventilation.

Evaluation of cerebral blood flow and hemodynamic reserve in symptomatic moyamoya disease using stable Xenon-CT blood flow

Moyamoya disease is a vascular abnormality seen in children and adults characterized by progressive narrowing of the internal carotid, middle, anterior, and posterior cerebral arteries and the development of leptomeningeal and proximal internal carotid artery collaterals, which appear diaphanous on angiogram. Although adults tend to present with subarachnoid hemorrhage and children with ischemic events, the clinical sequelae in these two populations overlap. Expanding upon work done at this institution using stable xenon computer tomographic blood flow determinations with acetazolamide and carbon dioxide challenge to predict which population of patients with severe carotid disease and hemodynamic compromise would benefit from surgical intervention, we used similar rationale to determine which patients with moyamoya disease would likely benefit from revascularization. Data and outcome concerning four such patients make up the body of this report.

Compromised cerebral blood flow reactivity is a predictor of stroke in patients with symptomatic carotid artery occlusive disease

PURPOSE

The purpose of this study was to determine whether the hemodynamic consequences of extracranial carotid disease correlate with the risk of subsequent cerebral infarction.

METHODS

In 95 patients with symptoms who had greater than or equal to 70% stenosis (31 patients) or who had occlusion (64 patients) of the ipsilateral carotid artery, cerebral blood flow was measured by the stable xenon/computed tomography technique both at baseline and after vasodilatory challenge with intravenous acetazolamide. Patients were stratified into group 1, 43 patients with no more than a 5% decrease in flow in any vascular territory, and group 2, 52 patients with greater than a 5% decrease in one or more vascular territories after an acetazolamide challenge.

RESULTS

In group 2, 15 (28.9%) of 52 patients had a new stroke, but only one (2.3%) of 43 patients in group 1 did (p = 0.0005). Of patients with total carotid occlusion 10 (26%) of 38 in group 2 and none (0%) of 26 in group 1 had a new stroke (p = 0.003). Of patients with greater than or equal to 70% stenosis, five (36%) of 14 in group 2 and only one (6%) of 17 in group 1 had a stroke (p = 0.067).

CONCLUSION

The loss of cerebral reactivity in patients with symptoms who had greater than or equal to 70% carotid stenosis or occlusion is an important predictor of impending cerebral infarction.

Correlation of xenon-enhanced computed tomography-defined cerebral blood flow reactivity and collateral flow patterns

BACKGROUND AND PURPOSE

A chronic compromise of cerebral hemodynamics has been shown to identify a group of patients at an increased risk for stroke. Because a "steal phenomenon" induced by a vasodilatory challenge has characterized the group at greatest risk, it was hypothesized that these individuals would also have a severe compromise of primary collaterals and an increased dependence on leptomeningeal collaterals.

METHODS

Twenty-three patients with symptomatic cerebrovascular disease underwent angiography and xenon-enhanced computed tomographic cerebral blood flow studies before and after 1 g IV acetazolamide within 6 months of each other. Cerebral blood flow vasoreactivity was classified by whether cerebral blood flow increased (> 5%) or was unchanged (+/- 5%) (group 1) or fell by > 5% (group 2) in any vascular territory. Angiographic collateralization was classified into four types: normal (type 1), willisian (type 2), ophthalmic (type 3), and leptomeningeal (type 4).

RESULTS

Twenty percent (2/10) of group 1 patients and 69% (9/13) of group 2 patients (P = .0009) had leptomeningeal collaterals.

CONCLUSIONS

A negative flow reactivity is significantly associated with a dependence on leptomeningeal collaterals and implies a state of maximal hemodynamic compromise.

Quantitative magnetic resonance imaging of human brain perfusion at 1.5 T using steady-state inversion of arterial water

We report our experience using a noninvasive magnetic resonance technique for quantitative imaging of human brain perfusion at 1.5 T. This technique uses magnetically inverted arterial water as a freely diffusible blood flow tracer. A perfusion image is calculated from magnetic resonance images acquired with and without arterial blood inversion and from an image of the apparent spin-lattice relaxation time. Single-slice perfusion maps were obtained from nine volunteers with approximately 1 x 2 x 5-mm resolution in an acquisition time of 15 min. Analysis yielded average perfusion rates of 93 +/- 16 ml.100 g-1.min-1 for gray matter, 38 +/- 10 ml.100 g-1.min-1 for white matter, and 52 +/- 8 ml.100 g-1.min-1 for whole brain. Significant changes in perfusion were observed during hyperventilation and breath holding. This technique may be used for quantitative measurement of perfusion in human brain without the risks and expense of methods which use exogenous tracers.

Mild hypothermia after cardiac arrest in dogs does not affect postarrest multifocal cerebral hypoperfusion

BACKGROUND AND PURPOSE

Although mild resuscitative hypothermia (34 degrees C) immediately after cardiac arrest improves neurological outcome in dogs, its effects on cerebral blood flow and metabolism are unknown.

METHODS

We used stable xenon-enhanced computed tomography to study local, regional, and global cerebral blood flow patterns up to 4 hours after cardiac arrest in dogs. We compared a normothermic (37.5 degrees C) control group (group I, n = 5) with a postarrest mild hypothermic group (group II, n = 5). After ventricular fibrillation of 12.5 minutes and reperfusion with brief cardiopulmonary bypass, the ventilation, normotension, normoxia, and mild hypocapnia were controlled to 4 hours after cardiac arrest. Group II received (minimal) head cooling during cardiac arrest, followed by systemic bypass cooling (to 34 degrees C) during the first hour of reperfusion after cardiac arrest.

RESULTS

The postarrest homogeneous transient hyperemia was followed by global hypoperfusion from 1 to 4 hours after arrest, with increased "no-flow" and "trickle-flow" voxels (compared with baseline), without group differences. At 1 to 4 hours, mean global cerebral blood flow in computed tomographic slices was 55% of baseline in group I and 64% in group II (NS). No flow (local cerebral blood flow < 5 mL/100 cm3 per minute) occurred in 5 +/- 2% of the voxels in group I versus 9 +/- 5% in group II (NS). Trickle flow (5 to 10 mL/100 cm3 per minute) occurred in 10 +/- 3% voxels in group I versus 16 +/- 4% in group II (NS). Cerebral blood flow values in eight brain regions followed the same hyperemia-hypoperfusion sequence as global cerebral blood flow, with no significant difference in regional values between groups. The global cerebral metabolic rate of oxygen, which ranged between 2.7 and 4.5 mL/100 cm3 per minute before arrest in both groups, was at 1 hour after arrest 1.8 +/- 0.3 mL in normothermic group I (n = 3) and 1.9 +/- 0.4 mL is still-hypothermic group II (n = 5); at 2 and 4 hours after arrest, it ranged between 1.2 and 4.2 mL in group I and between 1.2 and 2.6 mL in group II.

CONCLUSIONS

After cardiac arrest, mild resuscitative hypothermia lasting 1 hour does not significantly affect patterns of cerebral blood flow and oxygen uptake. This suggests that different mechanisms may explain its mitigating effect on brain damage.

Increased stroke risk predicted by compromised cerebral blood flow reactivity

The authors sought to determine risk for stroke in individuals with symptomatic carotid stenosis or occlusion based upon an assessment of cerebral blood flow (CBF) reserves. Vascular reserve was assessed by two consecutive xenon/computerized tomography (Xe/CT) CBF studies with intravenous acetazolamide introduced 20 minutes prior to the second study. Patients were assigned to one of two vasoreactivity groups. Group 2 included individuals who experienced a CBF reduction of more than 5% in at least one vascular territory and had a baseline flow of 45 cc/100 gm/min or less. Group 1 included all other individuals. Any territory with volume loss on CT of more than 50% was eliminated from analysis. Sixty-eight individuals were followed at 6-month intervals for a mean of 24 months. In Group 1 two strokes were observed contralateral to the side with lowest reserve, for a stroke incidence of 4.4%; in Group 2 eight strokes were observed ipsilateral to the side with lowest reserve, for a stroke incidence of 36%. The latter group had a 12.6 times greater chance of stroke (p = 0.0007). History of stroke, history of transient ischemic attacks, baseline CBF, and degree of stenosis were not associated with an increased stroke rate. In this study, significantly compromised vascular reserves accompanied by relatively low initial flow identified individuals who subsequently demonstrated a significantly increased rate of ipsilateral stroke.

Fast echo-shifted gradient-recalled MRI: combining a short repetition time with variable T2* weighting

The principles of a fast T2*-sensitized MR imaging method (Moonen et al., Magn. Reson. Med. 26, 184 (1992)) are extended to further increase T2* sensitivity. It is shown that the period of T2*-weighting can be lengthened by n TR-periods by appropriate gradient schemes without RF refocusing resulting in progressively delayed gradient-recalled echoes. This extension of the echo-shifting concept thus introduces large flexibility in the choice of T2*-weighting without changing total imaging time. The coherence pathway formalism is used to evaluate and describe the selection of the desired echo and the attenuation of unwanted coherences. The new techniques are demonstrated for tracking a bolus of susceptibility contrast agent in cat brain. Relative blood-volume maps are derived with expected contrast between white and gray matter.

Atraumatic quantitation of cerebral perfusion in cats by 19F magnetic resonance imaging

We have noninvasively produced low-resolution, quantitative nuclear magnetic resonance images of cerebral blood flow in 2-ml voxels in eight cats. Typical signal-to-noise of 4 to 1 was obtained in cerebral voxels in 16.5-s epochs. Mean flow during normocapnia (paCO2 = 39 +/- 4 mm Hg) and hypercapnia (paCO2 = 62 +/- 4 mm Hg) was 53 +/- 20 ml/100 g-min and 140 +/- 36 ml/100 g-min, respectively. Fast flows in normocapnia were 94 +/- 13 and 182 +/- 39 ml/100 g-min in hypercapnia. These results suggest that an atraumatic quantitative imaging assessment of cerebral perfusion may be possible in humans using these techniques.

The capillary network: a link between IVIM and classical perfusion

MR measurements based on motion encoding gradients, such as intravoxel incoherent motion imaging, could provide, in principle, information on flowing blood volume and blood velocity. This note shows that, in addition, the knowledge of the capillary network organization may provide a link between these measurements and those obtained by conventional and MR perfusion techniques based on tracer uptake by tissues.

Multifocal cerebral blood flow by Xe-CT and global cerebral metabolism after prolonged cardiac arrest in dogs. Reperfusion with open-chest CPR or cardiopulmonary bypass

Using the stable xenon-enhanced computed tomography (Xe-CT) method in dogs, we studied local, regional and global cerebral blood flow (LCBF, rCBF and gCBF) in two sham experiments and nine cardiac arrest experiments. Within the same experiments without arrest, gCBF and rCBF values were reproducible and stable. LCBF values varied over time. In group I (n = 4), ventricular fibrillation cardiac arrest (no blood flow) of 10 min was reversed by open-chest cardiopulmonary resuscitation (CPR). In group II (n = 5), ventricular fibrillation cardiac arrest of 12.5 min was reversed by brief closed-chest cardiopulmonary bypass. This was followed by controlled ventilation, normotension, normoxia, normocarbia and normothermia to 4 h (n = 7) or 20 h (n = 2) postarrest. The postarrest CBF patterns were similar in both groups. Open-chest CPR during ventricular fibrillation generated near-baseline gCBF and lower LCBF ranges. During postarrest spontaneous circulation, transient diffuse hyperemia was without low-flow regions, longer in brain stem and basal ganglia than in neocortex. During delayed hypoperfusion at 1-4 h postarrest (n = 9), mean gCBF was 44-60% baseline, rCBF in primarily gray matter regions was 15-49 ml/100 cm3 per min and LCBF voxels with trickle-flow and low-flow values, in percent of CT cut area, were increased over baseline. Global CMRO2 (n = 3 of group II) recovered to near baseline values between 1 and 4 h postarrest, while gCBF and O2 delivery were about 50% baseline (mismatching of O2 uptake and O2 delivery).

Dynamic heterogeneity of cerebral hypoperfusion after prolonged cardiac arrest in dogs measured by the stable xenon/CT technique: a preliminary study

After prolonged cardiac arrest and reperfusion, global cerebral blood flow (gCBF) is decreased to about 50% normal for many hours. Measurement of gCBF does not reveal regional variation of flow or permit testing of hypotheses involving multifocal no-flow or low-flow areas. We employed the noninvasive stable Xenon-enhanced Computerized Tomography (Xe/CT) local CBF (LCBF) method for use in dogs before and after ventricular fibrillation (VF) cardiac arrest of 10 min. This was followed by external cardiopulmonary resuscitation (CPR) and control of cardiovascular pulmonary variables to 7 h postarrest. In a sham (no arrest) experiment, the three CT levels studied showed normal regional heterogeneity of LCBF values, all between 10 and 75 ml/100 cm3 per min for white matter and 20 and 130 ml/100 cm3 per min for gray matter. In four preliminary CPR experiments, the expected global hyperemia at 15 min after arrest, was followed by hypoperfusion with gCBF reduced to about 50% control and increased heterogeneity of LCBF. Trickle flow areas (LCBF less than 10 ml/100 cm3 per min) not present prearrest, were interspersed among regions of low, normal, or even high flow. Regions of 125-500 mm3 with trickle flow or higher flows, in different areas at different times, involving deep and superficial structures migrated and persisted to 6 h, with gCBF remaining low. These preliminary results suggest: no initial no-reflow foci (less than 10 ml/100 cm3 per min) larger than 125 mm3 persisting through the initial global hyperemic phase; delayed multifocal hypoperfusion more severe than suggested by gCBF measurements; and trickle flow areas caused by dynamic factors.

Magnetic resonance imaging of perfusion using spin inversion of arterial water

A technique has been developed for proton magnetic resonance imaging (MRI) of perfusion, using water as a freely diffusable tracer, and its application to the measurement of cerebral blood flow (CBF) in the rat is demonstrated. The method involves labeling the inflowing water proton spins in the arterial blood by inverting them continuously at the neck region and observing the effects of inversion on the intensity of brain MRI. Solution to the Bloch equations, modified to include the effects of flow, allows regional perfusion rates to be measured from an image with spin inversion, a control image, and a T1 image. Continuous spin inversion labeling the arterial blood water was accomplished, using principles of adiabatic fast passage by applying continuous-wave radiofrequency power in the presence of a magnetic field gradient in the direction of arterial flow. In the detection slice used to measure perfusion, whole brain CBF averaged 1.39 +/- 0.19 ml.g-1.min-1 (mean +/- SEM, n = 5). The technique's sensitivity to changes in CBF was measured by using graded hypercarbia, a condition that is known to increase brain perfusion. CBF vs. pCO2 data yield a best-fit straight line described by CBF (ml.g-1.min-1) = 0.052pCO2 (mm Hg) - 0.173, in excellent agreement with values in the literature. Finally, perfusion images of a freeze-injured rat brain have been obtained, demonstrating the technique's ability to detect regional abnormalities in perfusion.

Hypertension with hemodilution prevents multifocal cerebral hypoperfusion after cardiac arrest in dogs

BACKGROUND

Improved neurological outcome with postarrest hypertensive hemodilution in an earlier study could be the result of more homogeneous cerebral perfusion and improved O2 delivery. We explored global, regional, and local cerebral blood flow by stable xenon-enhanced computed tomography and global cerebral metabolism in our dog cardiac arrest model.

METHODS

Ventricular fibrillation cardiac arrest of 12.5 minutes was reversed by brief cardiopulmonary bypass, followed by life support to 4 hours postarrest. We compared control group I (n = 5; mean arterial blood pressure, 100 mm Hg; hematocrit, greater than or equal to 35%) with immediately postarrest reflow-promoted group II (n = 5; mean arterial blood pressure, 140-110 mm Hg; hypervolemic hemodilution with plasma substitute to hematocrit, 20-25%).

RESULTS

After initial hyperemia in both groups, during the "delayed hypoperfusion phase" at 1-4 hours postarrest, global cerebral blood flow was 51-60% of baseline in group I versus 85-100% of baseline in group II (p less than 0.01). Percentages of brain tissue voxels with no flow, trickle flow, or low flow were lower (p less than 0.01) and mean regional cerebral blood flow values were higher in group II (p less than 0.01). Global cerebral oxygen uptake recovered to near baseline values at 3-4 hours postarrest in both groups. Postarrest arterial O2 content, however, in hemodiluted group II was 40-50% of that in group I. Thus, the O2 uptake/delivery ratio was increased (worsened) in both groups at 2-4 hours postarrest.

CONCLUSIONS

After prolonged cardiac arrest, immediately induced moderate hypertensive hemodilution to hematocrit 20-25% can normalize cerebral blood flow patterns (improve homogeneity of cerebral perfusion), but does not improve cerebral O2 delivery, since the flow benefit is offset by decreased arterial O2 content. Individualized titration of hematocrit or hemodilution with acellular O2 carrying blood substitute (stroma-free hemoglobin or fluorocarbon solution) would be required to improve O2 uptake/delivery ratio.

Effects of U74006F on multifocal cerebral blood flow and metabolism after cardiac arrest in dogs

Lipid peroxidation reactions during reperfusion after cardiac arrest may contribute to postischemic cerebral hypoperfusion, which in turn can contribute to permanent neurological dysfunction. We designed this study to determine whether the aminosteroid U74006F, a putative inhibitor of lipid peroxidation, mitigates cerebral multifocal hypoperfusion after cardiac arrest. We used our established dog model of ventricular fibrillation cardiac arrest (no blood flow) of 12.5 minutes, reperfusion by cardiopulmonary bypass of less than or equal to 5 minutes, and control of extracerebral variables during 4 hours postarrest. Cerebral blood flow was monitored by the stable xenon computed tomography method. Changes in cerebral oxygen consumption were obtained from mean blood flow values of coronal slices and the cerebral arteriovenous (sagittal sinus) oxygen content difference. A treatment group (n = 5) received U74006F starting with reperfusion (1.5 mg/kg i.a. plus 1.5 mg/kg i.v.) and three additional (graded) doses over 4 hours (total dose 4.5, 7.5, or 14.5 mg/kg). The U74006F-treated group showed the same postarrest transient hyperemia and protracted hypoperfusion in terms of global (computed tomography slice), regional, and local (multifocal) cerebral blood flow values and the same global cerebral oxygen consumption pattern as a concurrent control group (n = 5). At 1-4 hours postarrest, in both groups there was mismatching of global cerebral oxygen consumption, which reached baseline values, in relation to global cerebral blood flow and oxygen delivery, which remained at 50% of baseline. We conclude that treatment with U74006F after prolonged cardiac arrest causes no deleterious side effects and does not seem to alter multifocal postarrest cerebral blood flow and oxygen consumption.

Acute regional cerebral blood flow changes caused by severe head injuries

To evaluate the changes in cerebral blood flow (CBF) that occur immediately after head injury and the effects of different posttraumatic lesions on CBF, 61 CBF studies were obtained using the xenon-computerized tomography method in 32 severely head-injured adults (Glasgow Coma Scale score (GCS) less than or equal to 7). The measurements were made within 7 days after injury, 43% in the first 24 hours. During the 1st day, patients with an initial GCS score of 3 or 4 and no surgical mass had significantly lower flows than did those with a higher GCS score or mass lesions (p less than 0.05): in the first 1 to 4 hours, those without surgical mass lesions had a mean CBF of 27 cc/100 gm/min, which rose to 44 cc/100 gm/min by 24 hours. Patients without surgical mass lesions who died tended to have a lower global CBF than did those with better outcomes. Mass lesions were associated with a high global CBF and bihemispheric contusions with the lowest flows. By 24 hours after injury, global blood flow increased in groups that originally had low flows and decreased in those with very high initial flows, such that by 36 to 48 hours, most patients had CBF values between 32 and 55 cc/100 gm/min. Lobar, basal ganglion, and brain-stem blood flow values frequently differed by 25% or more from global averages. Brain-stem CBF varied the most but did not correlate with clinical signs of brain-stem dysfunction. Double studies were performed at two different pCO2 values in 10 patients with various posttraumatic lesions, and the CO2 vasoresponsivity was calculated. Abnormal CO2 vasoresponsivity was found with acute subdural hematomas and defuse cerebral swelling but not with epidural hematomas. In patients without surgical mass lesions, the findings suggest that CBF in the first few hours after injury is often low, followed by a hyperemic phase that peaks at 24 hours. Global CBF values vary widely depending on the type of traumatic brain injury, and brain-stem flow is often not accurately reflected by global CBF values. These findings underscore the need to define regional CBF abnormalities in victims of severe head injury if treatment is intended to prevent regional ischemia.

Effect of stable xenon on regional cerebral blood flow and the electroencephalogram in normal volunteers

We evaluated the effects of breathing 35% stable xenon in 65% oxygen on regional cerebral blood flow and the electroencephalogram in 20 normal volunteers. We measured blood flow in 32 brain regions over both hemispheres with the xenon-133 intravenous injection technique in two protocols. In the first protocol (n = 10), a baseline study was followed by a second study during 5 minutes of breathing stable xenon; in the other protocol (n = 8), the baseline study was followed by a second study after 5 minutes of breathing stable xenon. Two volunteers were excluded due to excessive movements during the inhalation of stable xenon. Some of the remaining 18 volunteers had varying alterations of consciousness accompanied by electroencephalogram changes. After stable xenon inhalation the electroencephalogram returned to normal within 2-3 minutes. During stable xenon inhalation mean +/- SD PECO2 dropped significantly from 39.4 +/- 4.4 to 33.3 +/- 5.4 mm Hg in the first protocol and from 39.4 +/- 2.6 to 34.8 +/- 4.1 mm Hg in the second protocol due to hyperventilation in 13 volunteers. Mean regional cerebral blood flow increased significantly by 13.5-25.4% without correction for PECO2. In the first protocol regional cerebral blood flow increased by greater than 12% in 11-14 (depending on the flow parameter) of the 20 hemispheres. In the second protocol regional cerebral blood flow increased by greater than 12% in 9-13 of the 16 hemispheres. We conclude that cautious interpretation is necessary in the assessment of regional cerebral blood flow with 35% xenon-enhanced computed tomography.

Functional magnetic resonance imaging in medicine and physiology

Magnetic resonance imaging (MRI) is a well-established diagnostic tool that provides detailed information about macroscopic structure and anatomy. Recent advances in MRI allow the noninvasive spatial evaluation of various biophysical and biochemical processes in living systems. Specifically, the motion of water can be measured in processes such as vascular flow, capillary flow, diffusion, and exchange. In addition, the concentrations of various metabolites can be determined for the assessment of regional regulation of metabolism. Examples are given that demonstrate the use of functional MRI for clinical and research purposes. This development adds a new dimension to the application of magnetic resonance to medicine and physiology.

Stable xenon-enhanced CT measurement of cerebral blood flow in reversible focal ischemia in baboons

When the lateral striate arteries of the baboon are temporarily occluded for either 20 or 60 minutes, a near-cessation of blood flow is followed by a dramatic, transient local increase in blood flow values. These findings are evident from serial xenon (Xe)-computerized tomography (CT) measurement of cerebral blood flow (CBF). In this study, 20 minutes of vessel occlusion resulted in brief (less than 1 hour) hyperemia, with no subsequent CT alteration and minimal random neuronal injury. Sixty minutes of occlusion resulted in a more prolonged hyperemia, a low-density area on CT images within 3 hours of reperfusion, and infarction of all cellular elements within the anterior lentiform nucleus. The Xe-CT method provides a sensitive, noninvasive technique for examining sequential alterations of CBF in small regions deep within the brain. This method of recording CBF also permits correlative studies of cerebral infarction, both clinically and experimentally, and allows reasonable inference about the probabilities of neuronal tissue damage with or without reperfusion.

Functional cerebral imaging by susceptibility-contrast NMR

In vivo measurement of cerebral physiology by dynamic contrast-enhanced NMR is demonstrated. Time-resolved images of the cerebral transit of paramagnetic contrast agent were acquired using a new ultrafast NMR imaging technique and a novel mechanism of image contrast based on microscopic changes in tissue magnetic susceptibility. Global hypercapnia in dogs was used to establish the relationship between susceptibility-induced signal change and brain blood volume, and the response of gray and white matter to this microvascular stimulus was measured.

Xenon-enhanced computed tomography compared with [14C]iodoantipyrine for normal and low cerebral blood flow states in baboons

The correlation between the acute, invasive diffusible [14C]iodoantipyrine technique for cerebral blood flow and the noninvasive xenon-enhanced computed tomographic method has been assessed by simultaneous measurements in the baboon. Blood flows in small tissue volumes (about 0.125 cm3) were directly compared in normal and low flow states. These studies demonstrate a statistically significant association between the two methods (p less than 0.001). Similar correlations were obtained by both the Kendall (tau) and the Spearman (r) methods (r = 0.67 to 0.92, n greater than or equal to 19 for each study). The problems and limitations of such correlations are discussed.

Perfusion imaging with NMR contrast agents

Knowledge of regional hemodynamics has widespread application for both physiological research and clinical assessment. Here we review the use of MR contrast agents to measure tissue perfusion. Two primary mechanisms of image contrast are discussed: relaxivity and susceptibility effects. Relaxivity effects result from dipolar enhancement of T1 and T2 rates. Because tissue T1 rates are intrinsically smaller, the dominant effect is shortening of T1 relaxation times. The second mechanism of image contrast is the variation in tissue magnetic field produced by heterogeneous distribution of high magnetic susceptibility agents. Quantitation of tissue perfusion requires a detailed understanding of the relation between contrast agent concentration and associated MR signal changes. Studies to date show a linear relationship between contrast agent concentration and rate change in most organs. The exact nature of this relationship in the dynamic setting of rapid contrast agent passage through the microcirculatory bed is less well established. If this relationship is known, tracer kinetic modeling can be used to calculate regional blood flow and blood volume. Data are presented which indicate that this approach is feasible, and suggest the potential of contrast-enhanced NMR for high resolution in vivo mapping of both physiology and anatomy.

Fluorocarbon-23 measure of cat cerebral blood flow by nuclear magnetic resonance

We employed fluorocarbon-23 (trifluoromethane) as a nuclear magnetic resonance gaseous indicator of cerebral blood flow in seven cats. Pulsed inhalation of this indicator and switching between two coils allowed the acquisition of both an arterial input and a cerebral response function, making possible multicompartmental curve fits to cerebral uptake and clearance data. The brain:blood partition coefficient for trifluoromethane was 0.9 for both gray and white matter. Fast-compartment blood flows were normal and showed appropriate CO2 reactivity. Slow-compartment blood flows did not demonstrate CO2 reactivity, probably because cranial as well as white-matter blood flows were lumped together in the slow compartment. Although cerebral blood flow was stable during administration of 60% trifluoromethane, the compound did prove to be a mild cardiac sensitizer to epinephrine in five cats.

Cerebral blood flow determination within the first 8 hours of cerebral infarction using stable xenon-enhanced computed tomography

Cerebral blood flow mapping with stable xenon-enhanced computed tomography (Xe/CT) was performed in conjunction with conventional computed tomography (CT) within the first 8 hours after the onset of symptoms in seven patients with cerebral infarction. Six patients had hemispheric infarctions, and one had a progressive brainstem infarction. Three patients with very low (less than 10 ml/100 g/min) blood flow in an anatomic area appropriate for the neurologic deficit had no clinical improvement by the time of discharge from the hospital; follow-up CT scans of these three patients confirmed infarction in the area of very low blood flow. Three patients with moderate blood flow reductions (15-45 ml/100 g/min) in the appropriate anatomic area had significant clinical improvement from their initial deficits and had normal follow-up CT scans. One patient studied 8 hours after stroke had increased blood flow (hyperemia) in the appropriate anatomic area and made no clinical recovery.

Stable xenon enhanced computed tomography in the study of clinical and pathologic correlates of focal ischemia in baboons

When the lateral striate arteries of baboons are occluded, an immediate cessation of blood flow followed by a transient, minimal restitution of flow occurs in that vascular distribution. These findings are evident from serial xenon/computed tomography cerebral blood flow imaging. In our study, infarction consistently accompanied arterial occlusion for 6 hours or more. The xenon/computed tomography method provides a sensitive, noninvasive technique for examining sequential alterations of cerebral blood flow in small regions deep within the brain. This methodology for recording cerebral blood flow permits correlative studies of cerebral infarction, clinically and experimentally, and allows reasonable inferences about the probabilities of neural tissue damage.

Effect of low concentration stable xenon on the EEG power spectrum

The effect on the central nervous system of inhaled stable xenon, at concentrations of 25%, 30% and 35%, was assessed by evaluating changes in power spectra computed on the electroencephalogram. Ten normal adult subjects were studied in a protocol designed as a repeated measures experiment. Synchronous changes in the EEG power spectra were observed with stable xenon inhalation. These changes were equivalent for symmetrical electrode pairs, but the time history of the changes differed depending on the cortical region being measure. This suggests regional effects of stable xenon inhalation on the mechanisms producing the EEG.

Measurement of cerebral blood flow during xenon inhalation as measured by the microspheres method

Measurements of cerebral blood flow (CBF) were performed using the microsphere technique in non-human primates (baboons) to assess the effect of non-radioactive xenon gas inhalation on CBF. Blood flows in small tissue volumes (approximately 1 cm3) were directly measured before and during the inhalation of xenon/oxygen gas mixtures. The results of these studies demonstrated that when inhaled in relatively high concentrations, xenon gas does increase CBF, but the changes are more global than tissue-specific. The problems and limitations of such evaluations are discussed.

Stable xenon CT blood flow mapping for evaluation of patients with extracranial-intracranial bypass surgery

Xenon computerized tomography (Xe CT) blood flow studies were conducted in 25 patients referred for a possible extracranial-intracranial bypass procedure for occlusive vascular disease in one or more extra- or intracranial vessels. These studies were helpful in selecting eight candidates for surgery. The Xe CT studies were performed at one or two brain levels using a prototype Xe CT system for measurement of cerebral blood flow which was designed in collaboration with the General Electric Co., and adapted for the GE 9800 scanner. In those patients selected to undergo operation, Xe CT demonstrated compromise of flow reserve regionally, globally, and/or in the watershed area. All eight patients who underwent the procedure showed a favorable clinical response postoperatively, and seven had a dramatic increase in flow. The 17 patients whose baseline CT studies showed no reduction of flow with the Xe CT method were not selected for surgery. All 25 patients have remained neurologically stable to date. Case studies of three of the eight patients undergoing bypass surgery are presented. This limited but consistent experience suggests that Xe CT blood flow mapping makes possible the recognition of brain regions in which flow reserves are compromised. This is due to the relatively high degree of spatial resolution that this technique provides and to the fact that mapping can be correlated directly with the anatomy. Used in combination with a careful clinical examination and an accurate medical history, this study method appears to be a useful guide in the selection of patients who are most at risk from hemodynamic instability and those who are most likely to benefit from flow-augmentation surgery.

Clinical experience with the use of xenon-enhanced CT blood flow mapping in cerebral vascular disease

Cerebral blood flow mapping with the xenon-enhanced/CT method has become a useful clinical tool in the management of patients with occlusive cerebral vascular disease. Studies involving 4-5 minutes of inhaling a xenon/oxygen mixture (less than or equal to 35%) can now be performed routinely with acceptable patient tolerance and compliance. Four cases with acute and chronic ischemic injuries are reported here to illustrate the manner in which this method has been used to characterize flow pattern in such patients and the relevance of this flow information to clinical patient management.